Question: What is the formula for calculating the focal length of a lens?

Options:

1. 1/f = 1/v + 1/u
2. f = v + u
3. f = v - u
4. 1/f = v - u

Correct Answer: 1/f = 1/v + 1/u

Solution:

The lens formula is given by 1/f = 1/v + 1/u, where f is the focal length, v is the image distance, and u is the object distance.